1. Field of the Invention
The present invention relates to a method and apparatus for image reading, and more particularly to a method and apparatus for image reading that detects foreign particles which disturb an image reading operation.
2. Discussion of the Background
Many image reading apparatuses perform various operations for adjusting an image signal from an image sensor before starting the image reading operation. For such an adjusting operation, the image reading apparatus is generally provided with a reference white plate for an image sensor to read a reference white image. Based on the reference white image data for one main scanning length obtained by reading the reference white plate, the image reading apparatus adjusts gains of image signal in order to justify the level of the image signal from the image sensor, or performs a shading correction to correct for signal distortion due to inaccurate optical mechanisms employed therein or variations in optoelectronic conversion components such as a CCD (charge-coupled device), and so forth.
Also, the image reading apparatus is provided with a reference black plate for an image sensor to read a reference black image. Based on the reference black image data for one main scanning length obtained by reading the reference black plate, the image reading apparatus reduces an offset which is previously included in an image signal from the image sensor.
To perform the above-described adjustment and correction, the image reading apparatus is required to read the reference white and reference black plates in an accurate manner. However, even if the image reading apparatus reads the reference white and reference black plates, the reading accuracy may be disturbed by a foreign particle such as dust or the like which is put on the reference white and reference black plates.
More specifically, if a dust particle which is less white is put on the reference white image of the reference white plate, the level of the reference white image reading at the location of the foreign particle may erroneously be decreased. If a dust particle which is less black is put on the reference black image of the reference black plate, the level of the reference black image reading at the location of the foreign particle may erroneously be increased. The image reading apparatus will adjust the image signal based on such an erroneous reference white/black image data. When the image reading apparatus is connected to a printer, for example, the image data may be recorded on a recording sheet with an erroneous black line or the like.
Various solutions for the above-described problem have been introduced. For example, Japanese Patent Publication No. 60-251768(1985) describes a technique in which the image sensor reads a number of times reference white and reference black images in order to reduce an adverse influence to the reference white and reference black image signals by the foreign particle. However, this technique fails in speed, requiring a relatively long time period for completing the adjustment. In fact, causes of the erroneous image data may not be only the foreign particles put on the reference white/black image plate but also those on the interior of the image sensor. The former can be recovered by removing the foreign particles but the latter may need an exchange of the image sensor, for example.
Typically, these problems caused by the inner or outer foreign particles are found only after outputting the image data in a visual form such as a printout form. Yet, an abnormal print generally needs to be analyzed as to what the cause is. The foreign particle may be only one of the causes.
Therefore, at present, there is no image reading apparatus which is capable of detecting a foreign particle such as dust or the like which is put on a reference white/black plate or an image sensor, which disturbs an image reading operation.